YoanaRuseva

JackReed

RüdigerLandmann

DonDomingo

This document explains how to manage power consumption on Red Hat Enterprise Linux 6 systems effectively. The following sections discuss different techniques that lower power consumption (for both server and laptop), and how each technique affects the overall performance of your system.

Chapter 1. Overview

Power management has been one of our focus points for improvements for Red Hat Enterprise Linux 6. Limiting the power used by computer systems is one of the most important aspects of green IT (environmentally friendly computing), a set of considerations that also encompasses the use of recyclable materials, the environmental impact of hardware production, and environmental awareness in the design and deployment of systems. In this document, we provide guidance and information regarding power management of your systems running Red Hat Enterprise Linux 6.

1.1. Importance of Power Management

At the core of power management is an understanding of how to effectively optimize energy consumption of each system component. This entails studying the different tasks that your system performs, and configuring each component to ensure that its performance is just right for the job.

As a rule, lowering the power consumption of a specific component (or of the system as a whole) will lead to lower heat and naturally, performance. As such, you should thoroughly study and test the decrease in performance afforded by any configurations you make, especially for mission-critical systems.

By studying the different tasks that your system performs, and configuring each component to ensure that its performance is just sufficient for the job, you can save energy, generate less heat, and optimize battery life for laptops. Many of the principles for analysis and tuning of a system in regard to power consumption are similar to those for performance tuning. To some degree, power management and performance tuning are opposite approaches to system configuration, because systems are usually optimized either towards performance or power. This manual describes the tools that Red Hat provides and the techniques we have developed to help you in this process.

Red Hat Enterprise Linux 6 already comes with a lot of new power management features that are enabled by default. They were all selectively chosen to not impact the performance of a typical server or desktop use case. However, for very specific use cases where maximum throughput, lowest latency, or highest CPU performance is absolutely required, a review of those defaults might be necessary.

To decide whether you should optimize your machines using the techniques described in this document, ask yourself a few questions:

The importance of power optimization depends on whether your company has guidelines that need to be followed or if there are any regulations that you have to fulfill.

Q:

How much do I need to optimize?

A:

Several of the techniques we present do not require you to go through the whole process of auditing and analyzing your machine in detail but instead offer a set of general optimizations that typically improve power usage. Those will of course typically not be as good as a manually audited and optimized system, but provide a good compromise.

Q:

Will optimization reduce system performance to an unacceptable level?

A:

Most of the techniques described in this document impact the performance of your system noticeably. If you choose to implement power management beyond the defaults already in place in Red Hat Enterprise Linux 6, you should monitor the performance of the system after power optimization and decide if the performance loss is acceptable.

Q:

Will the time and resources spent to optimize the system outweigh the gains achieved?

A:

Optimizing a single system manually following the whole process is typically not worth it as the time and cost spent doing so is far higher than the typical benefit you would get over the lifetime of a single machine. On the other hand if you for example roll out 10000 desktop systems to your offices all using the same configuration and setup then creating one optimized setup and applying that to all 10000 machines is most likely a good idea.

The following sections will explain how optimal hardware performance benefits your system in terms of energy consumption.

1.2. Power Management Basics

Effective power management is built on the following principles:

An idle CPU should only wake up when needed

The Red Hat Enterprise Linux 5 kernel used a periodic timer for each CPU. This timer prevents the CPU from truly going idle, as it requires the CPU to process each timer event (which would happen every few milliseconds, depending on the setting), regardless of whether any process was running or not. A large part of effective power management involves reducing the frequency at which CPU wakeups are made.

Because of this, the Linux kernel in Red Hat Enterprise Linux 6 eliminates the periodic timer: as a result, the idle state of a CPU is now tickless. This prevents the CPU from consuming unnecessary power when it is idle. However, benefits from this feature can be offset if your system has applications that create unnecessary timer events. Polling events (such as checks for volume changes, mouse movement, and the like) are examples of such events.

This is especially true for devices that have moving parts (for example, hard disks). In addition to this, some applications may leave an unused but enabled device "open"; when this occurs, the kernel assumes that the device is in use, which can prevent the device from going into a power saving state.

Low activity should translate to low wattage

In many cases, however, this depends on modern hardware and correct BIOS configuration. Older system components often do not have support for some of the new features that we now can support in Red Hat Enterprise Linux 6. Make sure that you are using the latest official firmware for your systems and that in the power management or device configuration sections of the BIOS the power management features are enabled. Some features to look for include:

SpeedStep

PowerNow!

Cool'n'Quiet

ACPI (C state)

Smart

If your hardware has support for these features and they are enabled in the BIOS, Red Hat Enterprise Linux 6 will use them by default.

Different forms of CPU states and their effects

Modern CPUs together with Advanced Configuration and Power Interface (ACPI) provide different power states. The three different states are:

Sleep (C-states)

Frequency (P-states)

Heat output (T-states or "thermal states")

A CPU running on the lowest sleep state possible consumes the least amount of watts, but it also takes considerably more time to wake it up from that state when needed. In very rare cases this can lead to the CPU having to wake up immediately every time it just went to sleep. This situation results in an effectively permanently busy CPU and loses some of the potential power saving if another state had been used.

A turned off machine uses the least amount of power

As obvious as this might sound, one of the best ways to actually save power is to turn off systems. For example, your company can develop a corporate culture focused on "green IT" awareness with a guideline to turn of machines during lunch break or when going home. You also might consolidate several physical servers into one bigger server and virtualize them using the virtualization technology we ship with Red Hat Enterprise Linux 6.

Chapter 2. Power management auditing and analysis

2.1. Audit and analysis overview

The detailed manual audit, analysis, and tuning of a single system is usually the exception because the time and cost spent to do so typically outweighs the benefits gained from these last pieces of system tuning. However, performing these tasks once for a large number of nearly identical systems where you can reuse the same settings for all systems can be very useful. For example, consider the deployment of thousands of desktop systems, or a HPC cluster where the machines are nearly identical. Another reason to do auditing and analysis is to provide a basis for comparison against which you can identify regressions or changes in system behavior in the future. The results of this analysis can be very helpful in cases where hardware, BIOS, or software updates happen regularly and you want to avoid any surprises with regard to power consumption. Generally, a thorough audit and analysis gives you a much better idea of what is really happening on a particular system.

Auditing and analyzing a system with regard to power consumption is relatively hard, even with the most modern systems available. Most systems do not provide the necessary means to measure power use via software. Exceptions exist though: the ILO management console of Hewlett Packard server systems has a power management module that you can access through the web. IBM provides a similar solution in their BladeCenter power management module. On some Dell systems, the IT Assistant offers power monitoring capabilities as well. Other vendors are likely to offer similar capabilities for their server platforms, but as can be seen there is no single solution available that is supported by all vendors. If your system has no inbuilt mechanism to measure power consumption, a few other choices exist. You could install a special power supply for your system that offers power consumption information through USB. The Gigabyte Odin GT 550 W PC power supply is one such example. As a last resort, some external watt meters like the Watts up? PRO have a USB connector.

Direct measurements of power consumption is often only necessary to maximize savings as far as possible. Fortunately, other means are available to measure if changes are in effect or how the system is behaving. This chapter describes the necessary tools.

2.2. PowerTOP

The introduction of the tickless kernel in Red Hat Enterprise Linux 6 (refer to Section 3.6, “Tickless Kernel”) allows the CPU to enter the idle state more frequently, reducing power consumption and improving power management. The PowerTOP tool identifies specific components of kernel and userspace applications that frequently wake up the CPU. PowerTOP was used in development to perform the audits described in Section 3.13, “Optimizations in User Space” that led to many applications being tuned in this release, reducing unnecessary CPU wake up by a factor of ten.

Red Hat Enterprise Linux 6 comes with version 2.x of PowerTOP. This version is a complete rewrite of the 1.x codebase. It features a clearer tab-based user interface and extensively uses the kernel "perf" infrastructure to give more accurate data. The power behavior of system devices is tracked and prominently displayed, so problems can be pinpointed quickly. More experimentally, the 2.x codebase includes a power estimation engine that can indicate how much power individual devices and processes are consuming. Refer to Figure 2.1, “PowerTOP in Operation”.

To install PowerTOP run, as root, the following command:

yum install powertop

To run PowerTOP, execute the following command as root:

powertop

PowerTOP can provide an estimate of the total power usage of the system and show individual power usage for each process, device, kernel work, timer, and interrupt handler. Laptops should run on battery power during this task. To calibrate the power estimation engine, run, as root, the following command:

powertop --calibrate

Calibration takes time. The process performs various tests, and will cycle through brightness levels and switch devices on and off. Allow the process to finish and do not interact with the machine during the calibration. When it completes, PowerTOP starts as normal. Then keep PowerTOP running for approximately an hour to collect data. When enough data is collected, power estimation figures will be displayed in the first column.

If you are executing powertop --calibrate on a laptop, it should still be running on battery power so that all available data is presented.

While it runs, PowerTOP gathers statistics from the system. In the Overview tab, you can view a list of the components that are either sending wake-ups to the CPU most frequently or are consuming the most power (refer to Figure 2.1, “PowerTOP in Operation”). The adjacent columns display:

power estimation

how the resource is being used

wakeups per second

the classification of the component, such as process, device, or timer

description of the component

Wakeups per second indicates how efficiently the services or the devices and drivers of the kernel are performing. Less wakeups means less power is consumed. Components are ordered by how much further their power usage can be optimized.

Tuning driver components typically requires kernel changes, which is beyond the scope of this document. However, userland processes that send wakeups are more easily managed. First, determine whether this service or application needs to run at all on this system. If not, simply deactivate it. To turn off an old System V service permanently, run:

chkconfig servicename.service off

For more details about the process, run, as root, the following commands:

ps -awux | grep processnamestrace -p processid

If the trace looks like it is repeating itself, then it probably is a busy loop. Fixing such bugs typically requires a code change in that component.

As seen in Figure 2.1, “PowerTOP in Operation”, total power consumption and the remaining battery life are displayed, if applicable. Below these is a short summary featuring total wakeups per second, GPU operations per second, and virtual filesystem operations per second. In the rest of the screen there is a list of processes, interrupts, devices and other resources sorted according to their utilization. If properly calibrated, a power consumption estimation for every listed item in the first column is shown as well.

Use the Tab and Shift+Tab keys to cycle through tabs. In the Idle stats tab, use of C-states is shown for all processors and cores. In the Frequency stats tab, use of P-states including the Turbo mode (if applicable) is shown for all processors and cores. The longer the CPU stays in the higher C- or P-states, the better (C4 being higher than C3). This is a good indication of how well the CPU usage has been optimized. Residency should ideally be 90% or more in the highest C- or P-state while the system is idle.

The Device Stats tab provides similar information to the Overview tab but only for devices.

The Tunables tab contains suggestions for optimizing the system for lower power consumption. Use the up and down keys to move through suggestions and the enter key to toggle the suggestion on and off.

Figure 2.1. PowerTOP in Operation

You can also generate HTML reports by running PowerTOP with the --html option. Replace the htmlfile.html parameter with the desired name for the output file:

powertop --html=htmlfile.html

By default PowerTOP takes measurements in 20 seconds intervals, you can change it with the --time option:

PowerTOP can also be used along with the turbostat utility. It is a reporting tool that displays information about processor topology, frequency, idle power-state statistics, temperature, and power usage on Intel 64 processors. For more information about turbostat, refer to the turbostat man page, or the relevant section in Performance Tuning Guide.

2.3. diskdevstat and netdevstat

diskdevstat and netdevstat are SystemTap tools that collect detailed information about the disk activity and network activity of all applications running on a system. These tools were inspired by PowerTOP, which shows the number of CPU wakeups by every application per second (refer to Section 2.2, “PowerTOP”). The statistics that these tools collect allow you to identify applications that waste power with many small I/O operations rather than fewer, larger operations. Other monitoring tools that measure only transfer speeds do not help to identify this type of usage.

Install these tools with SystemTap with the following command as root:

These three applications have a WRITE_CNT greater than 0, which means that they performed some form of write during the measurement. Of those, plasma was the worst offender by a large degree: it performed the most write operations, and the average time between writes was the lowest. Plasma would therefore be the best candidate to investigate if you were concerned about power-inefficient applications.

Use the strace and ltrace commands to examine applications more closely by tracing all system calls of the given process ID. Run:

strace -p 2789

The output of strace contains a repeating pattern every 45 seconds that opened the KDE icon cache file of the user for writing followed by an immediate close of the file again. This led to a necessary physical write to the hard disk as the file metadata (specifically, the modification time) had changed. The final fix was to prevent those unnecessary calls when no updates to the icons had occurred.

For reference on what the columns in the diskdevstat command stand for, see this table:

Table 2.1. Reading the diskdevstat Output

PID

the process ID of the application

UID

the user ID under which the applications is running

DEV

the device on which the I/O took place

WRITE_CNT

the total number of write operations

WRITE_MIN

the lowest time taken for two consecutive writes (in seconds)

WRITE_MAX

the greatest time taken for two consecutive writes (in seconds)

WRITE_AVG

the average time taken for two consecutive writes (in seconds)

READ_CNT

the total number of read operations

READ_MIN

the lowest time taken for two consecutive reads (in seconds)

READ_MAX

the greatest time taken for two consecutive reads (in seconds)

READ_AVG

the average time taken for two consecutive reads (in seconds)

COMMAND

the name of the process

2.4. Battery Life Tool Kit

Red Hat Enterprise Linux 6 introduces the Battery Life Tool Kit (BLTK), a test suite that simulates and analyzes battery life and performance. BLTK achieves this by performing sets of tasks that simulate specific user groups and reporting on the results. Although developed specifically to test notebook performance, BLTK can also report on the performance of desktop computers when started with the -a option.

BLTK allows you to generate very reproducible workloads that are comparable to real use of a machine. For example, the office workload writes a text, corrects things in it, and does the same for a spreadsheet. Running BLTK combined with PowerTOP or any of the other auditing or analysis tool allows you to test if the optimizations you performed have an effect when the machine is actively in use instead of only idling. Because you can run the exact same workload multiple times for different settings, you can compare results for different settings.

Install BLTK with the command:

yum install bltk

The bltk command is run with various workloads to simulate different types of laptop usage, and one of the options. See the format of the command:

bltk workloadoptions

The workload options available by default are:

-I, --idle

system is idle, to use as a baseline for comparison with other workloads

the time (in seconds) over which to run the test; use this option with the idle workload

-F filename, --file filename

specifies a file to be used by a particular workload, for example, a file for the player workload to play instead of accessing the CD or DVD drive

-W application, --prog application

specifies an application to be used by a particular workload, for example, a browser other than Firefox for the reader workload

Example 2.2. Measuring Battery Life and Performance under Idle Workload

To run the idle workload for 120 seconds, run:

bltk -I -T 120

BLTK supports a large number of more specialized options. For details, refer to the bltk(1) man page.

BLTK saves the results it generates in a directory specified in the /etc/bltk.conf configuration file — by default, ~/.bltk/workload.results.number/. For example, the ~/.bltk/reader.results.002/ directory holds the results of the third test with the reader workload (the first test is not numbered). The results are spread across several text files. To condense these results into a format that is easy to read, run:

bltk_report path_to_results_directory

The results now appear in a text file named Report in the results directory. To view the results in a terminal emulator instead, use the -o option:

bltk_report -o path_to_results_directory

2.5. tuned and ktune

tuned is a daemon that monitors the use of system components and dynamically tunes system settings based on that monitoring information. Dynamic tuning accounts for the way that various system components are used differently throughout the uptime for any given system. For example, the hard drive is used heavily during startup and login, but is barely used later when a user might mainly work with applications like OpenOffice or email clients. Similarly, the CPU and network devices are used differently at different times. tuned monitors the activity of these components and reacts to changes in their use.

As a practical example, consider a typical office workstation. Most of the time, the Ethernet network interface will be very inactive. Only a few emails will go in and out every once in a while or some web pages might be loaded. For those kinds of loads, the network interface does not have to run at full speed all the time, as it does by default. tuned has a monitoring and tuning plugin for network devices that can detect that low activity and then automatically lower the speed of that interface, typically resulting in lower power usage. If activity on the interface increases drastically for a longer period of time, for example because a DVD image is being downloaded or an email with a large attachment is opened, tuned detects this and sets the interface speed to maximum to offer the best performance while the activity level is so high. This principle is used for the other plugins for CPU and hard disks as well.

Network devices are not configured to behave this way by default because speed changes can take several seconds to take effect and therefore directly and visibly impact the user experience. Similar considerations apply for the CPU and hard drive tuning plugins. When a hard drive has been spun down, it can take several seconds for it to spin up again which results in an observed lack of responsiveness of the system during that period. The latency side effect is smallest for the CPU plugin, but it is still at least measurable, though hardly noticeable by a user.

Alongside of tuned, Red Hat Enterprise Linux 6 also offers ktune. ktune was introduced in Red Hat Enterprise Linux 5.3 as a framework and service to optimize the performance of a machine for a specific use cases. Since then, ktune has improved to such a degree that we now use it as the static part of our general tuning framework. It is mainly used in the different predefined profiles described in Section 2.5.2, “Tuned-adm”.

Install the tuned package and its associated systemtap scripts with the command:

yum install tuned

Installing the tuned package also sets up a sample configuration file at /etc/tuned.conf and activates the default profile.

Start tuned by running:

service tuned start

To start tuned every time the machine boots, run:

chkconfig tuned on

tuned itself has additional options that you can use when you run it manually. The available options are:

-d, --daemon

start tuned as a daemon instead of in the foreground.

-c, --conffile

use a configuration file with the specified name and path, for example, --conffile=/etc/tuned2.conf. The default is /etc/tuned.conf.

-D, --debug

use the highest level of logging.

2.5.1. The tuned.conf file

The tuned.conf file contains configuration settings for tuned. By default, it is located at /etc/tuned.conf, but you can specify a different name and location by starting tuned with the --conffile option.

The config file must always contain a [main] section that defines the general parameters for tuned. The file then contains a section for each plugin.

The [main] section contains the following options:

interval

the interval at which tuned should monitor and tune the system, in seconds. The default value is 10.

verbose

specifies whether output should be verbose. The default value is False.

logging

specifies the minimum priority of messages to be logged. In descending order, allowable values are: critical, error, warning, info, and debug. The default value is info.

logging_disable

specifies the maximum priority of messages to be logged; any messages with this priority or lower will not be logged. In descending order, allowable values are: critical, error, warning, info, and debug. The value notset disables this option.

Each plugin has its own section, specified with the name of the plugin in square brackets; for example: [CPUTuning]. Each plugin can have its own options, but the following apply to all plugins:

enabled

specifies whether the plugin is enabled or not. The default value is True.

verbose

specifies whether output should be verbose. If not set for this plugin, the value is inherited from [main].

logging

specifies the minimum priority of messages to be logged. If not set for this plugin, the value is inherited from [main].

2.5.2. Tuned-adm

Often, a detailed audit and analysis of a system can be very time consuming and might not be worth the few extra watts you might be able to save by doing so. Previously, the only alternative was simply to use the defaults. Therefore, Red Hat Enterprise Linux 6 includes separate profiles for specific use cases as an alternative between those two extremes, together with the tuned-adm tool that allows you to switch between these profiles easily at the command line. Red Hat Enterprise Linux 6 includes a number of predefined profiles for typical use cases that you can simply select and activate with the tuned-adm command, but you can also create, modify or delete profiles yourself.

To list all available profiles and identify the current active profile, run:

tuned-adm list

To only display the currently active profile, run:

tuned-adm active

To switch to one of the available profiles, run:

tuned-adm profile profile_name

Example 2.4. Switching to the server-powersave profile

tuned-adm profile server-powersave

To disable all tuning:

tuned-adm off

When you first install tuned, the default profile will be active. Red Hat Enterprise Linux 6 also includes the following predefined profiles:

default

The default power-saving profile. It has the lowest impact on power saving of the available profiles and only enables CPU and disk plugins of tuned.

A medium-impact power-saving profile directed at laptops running on AC. Enables ALPM powersaving for SATA host adapters, WiFi power saving, as well as the CPU, Ethernet and disk plugins of tuned.

laptop-battery-powersave

A high-impact power-saving profile directed at laptops running on battery. It activates all power saving mechanisms from the previous profiles plus it enables the multi-core power-savings scheduler for low wakeup systems and makes sure that the ondemand governor is active and that AC97 audio power-saving is enabled. You can use this profile to save the maximum amount of power on any kind of system, not only laptops on battery power. The tradeoff for this profile is a noticeable impact on performance, specifically latency of disk and network I/O.

A server profile for typical throughput performance tuning. It disables tuned and ktune power saving mechanisms, enables sysctl settings that improve the throughput performance of your disk and network I/O, and switches to the deadline scheduler. CPU governor is set to performance.

latency-performance

A server profile for typical latency performance tuning. This profile disables dynamic tuning mechanisms and transparent hugepages. It uses the performance governer for p-states through cpuspeed, and sets the I/O scheduler to deadline. Additionally, in Red Hat Enterprise Linux 6.5 and later, the profile requests a cpu_dma_latency value of 1. In Red Hat Enterprise Linux 6.4 and earlier, cpu_dma_latency requested a value of 0.

This profile is optimized for virtual machines. It is based on the enterprise-storage profile, but also decreases the swappiness of virtual memory. This profile is available in Red Hat Enterprise Linux 6.3 and later.

virtual-host

Based on the enterprise-storage profile, this profile decreases the swappiness of virtual memory and enables more aggressive writeback of dirty pages. Non-root and non-boot file systems are mounted with barrier=0. Additionally, as of Red Hat Enterprise Linux 6.5, the kernel.sched_migration_cost parameter is set to 5 milliseconds. Prior to Red Hat Enterprise Linux 6.5, kernel.sched_migration_cost used the default value of 0.5 milliseconds

oracle

A profile optimized for Oracle databases loads based on throughput-performance profile. It additionaly disables transparent huge pages and modifies some other performance related kernel parameters. This profile is provided by tuned-profiles-oracle package. It is available in Red Hat Enterprise Linux 6.8 and later.

Note

There may be more product specific or 3rd party Tuned profiles available. Such profiles are usually provided by separate RPM packages.

All the profiles are stored in separate subdirectories under /etc/tune-profiles. Thus /etc/tune-profiles/desktop-powersave contains all the necessary files and settings for the desktop-powersave profile. Each of these directories contains up to four files:

tuned.conf

the configuration for the tuned service to be active for this profile.

sysctl.ktune

the sysctl settings used by ktune. The format is identical to the /etc/sysconfig/sysctl file (refer to the sysctl and sysctl.conf man pages).

an init-style shell script used by the ktune service which can run specific commands during system startup to tune the system.

Example 2.5. Creating a New Profile

The easiest way to start a new profile is to copy an existing one and edit the configuration options. The laptop-battery-powersave profile contains a very rich set of tunings already and is therefore a useful starting point.

Modify any of the files in the new profile to match your personal requirements. If you require the detection of CD changes, you could disable that optimization by commenting out the appropriate line in the ktune.sh script:

2.6. DeviceKit-power and devkit-power

In Red Hat Enterprise Linux 6 DeviceKit-power assumes the power management functions that were part of HAL and some of the functions that were part of GNOME Power Manager in previous releases of Red Hat Enterprise Linux (refer also to Section 2.7, “GNOME Power Manager”). DeviceKit-power provides a daemon, an API, and a set of command-line tools. Each power source on the system is represented as a device, whether it is a physical device or not. For example, a laptop battery and an AC power source are both represented as devices.

You can access the command-line tools with the devkit-power command and the following options:

monitors the system for changes to power devices, for example, the connection or disconnection of a source of AC power, or the depletion of a battery. Press Ctrl+C to stop monitoring the system.

--monitor-detail

monitors the system for changes to power devices, for example, the connection or disconnection of a source of AC power, or the depletion of a battery. The --monitor-detail option presents more detail than the --monitor option. Press Ctrl+C to stop monitoring the system.

--show-info object_path, -i object_path

displays all information available for a particular object path.

Example 2.7. Using the -i option

To obtain information about a battery on your system represented by the object path /org/freedesktop/UPower/DeviceKit/power/battery_BAT0, run:

devkit-power -i /org/freedesktop/UPower/DeviceKit/power/battery_BAT0

2.7. GNOME Power Manager

GNOME Power Manager is a daemon that is installed as part of the GNOME desktop. Much of the power-management functionality that GNOME Power Manager provided in earlier versions of Red Hat Enterprise Linux has become part of DeviceKit-power in Red Hat Enterprise Linux 6 (refer to Section 2.6, “DeviceKit-power and devkit-power”. However, GNOME Power Manager remains a front end for that functionality. Through an applet in the system tray, GNOME Power Manager notifies you of changes in your system's power status; for example, a change from battery to AC power. It also reports battery status, and warns you when battery power is low.

GNOME Power Manager also allows you to configure some basic power management settings. To access these settings, click the GNOME Power Manager icon in the system tray, then click Preferences

The Power Management Preferences screen contains two tabs:

On AC Power

General

On a laptop, Power Management Preferences will contain a third tab:

On Battery Power

Use the On AC Power and On Battery Power tabs to specify how much time must pass before the display is turned off on an inactive system, how much time must pass before an inactive system is put to sleep, and whether the system should spin down hard disks when not in use. The On Battery Power tab also allows you to set the display brightness and to choose a behavior for a system with a critically low battery. For example, by default, GNOME Power Manager makes a system hibernate when its battery level reaches a critically low level. Use the General tab to set behaviours for the (physical) power button and suspend button on your system, and to specify the circumstances under which the GNOME Power Manager icon should appear in the system tray.

2.8. Other means for auditing

Red Hat Enterprise Linux 6 offers quite a few more tools with which to perform system auditing and analysis. Most of them can be used as a supplementary source of information in case you want to verify what you have discovered already or in case you need more in-depth information on certain parts. Many of these tools are used for performance tuning as well. They include:

vmstat

vmstat gives you detailed information about processes, memory, paging, block I/O, traps, and CPU activity. Use it to take a closer look at what the system overall does and where it is busy.

iostat

iostat is similar to vmstat, but only for I/O on block devices. It also provides more verbose output and statistics.

blktrace

blktrace is a very detailed block I/O trace program. It breaks down information to single blocks associated with applications. It is very useful in combination with diskdevstat.

Chapter 3. Core Infrastructure and Mechanics

Important

To use the cpupower command featured in this chapter, ensure you have the cpupowerutils package installed.

3.1. CPU Idle States

CPUs with the x86 architecture support various states in which parts of the CPU are deactivated or run at lower performance settings. These states, known as C-states, allow systems to save power by partially deactivating CPUs that are not in use. C-states are numbered from C0 upwards, with higher numbers representing decreased CPU functionality and greater power saving. C-States of a given number are broadly similar across processors, although the exact details of the specific feature sets of the state may vary between processor families. C-States 0–3 are defined as follows:

C0

the operating or running state. In this state, the CPU is working and not idle at all.

C1, Halt

a state where the processor is not executing any instructions but is typically not in a lower power state. The CPU can continue processing with practically no delay. All processors offering C-States need to support this state. Pentium 4 processors support an enhanced C1 state called C1E that actually is a state for lower power consumption.

C2, Stop-Clock

a state where the clock is frozen for this processor but it keeps the complete state for its registers and caches, so after starting the clock again it can immediately start processing again. This is an optional state.

C3, Sleep

a state where the processor really goes to sleep and does not need to keep its cache up to date. Waking up from this state takes considerably longer than from C2 due to this. Again this is an optional state.

To view available idle states and other statistics for the CPUidle driver, run the following command:

cpupower idle-info

Recent Intel CPUs with the "Nehalem" microarchitecture feature a new C-State, C6, which can reduce the voltage supply of a CPU to zero, but typically reduces power consumption by between 80% and 90%. The kernel in Red Hat Enterprise Linux 6 includes optimizations for this new C-State.

3.2. Using CPUfreq Governors

One of the most effective ways to reduce power consumption and heat output on your system is to use CPUfreq. CPUfreq — also referred to as CPU speed scaling — allows the clock speed of the processor to be adjusted on the fly. This enables the system to run at a reduced clock speed to save power. The rules for shifting frequencies, whether to a faster or slower clock speed, and when to shift frequencies, are defined by the CPUfreq governor.

The governor defines the power characteristics of the system CPU, which in turn affects CPU performance. Each governor has its own unique behavior, purpose, and suitability in terms of workload. This section describes how to choose and configure a CPUfreq governor, the characteristics of each governor, and what kind of workload each governor is suitable for.

3.2.1. CPUfreq Governor Types

This section lists and describes the different types of CPUfreq governors available in Red Hat Enterprise Linux 6.

cpufreq_performance

The Performance governor forces the CPU to use the highest possible clock frequency. This frequency will be statically set, and will not change. As such, this particular governor offers no power saving benefit. It is only suitable for hours of heavy workload, and even then only during times wherein the CPU is rarely (or never) idle.

cpufreq_powersave

By contrast, the Powersave governor forces the CPU to use the lowest possible clock frequency. This frequency will be statically set, and will not change. As such, this particular governor offers maximum power savings, but at the cost of the lowest CPU performance.

The term powersave can sometimes be deceiving, though, since (in principle) a slow CPU on full load consumes more power than a fast CPU that is not loaded. As such, while it may be advisable to set the CPU to use the Powersave governor during times of expected low activity, any unexpected high loads during that time can cause the system to actually consume more power.

The Powersave governor is, in simple terms, more of a speed limiter for the CPU than a power saver. It is most useful in systems and environments where overheating can be a problem.

cpufreq_ondemand

The Ondemand governor is a dynamic governor that allows the CPU to achieve maximum clock frequency when system load is high, and also minimum clock frequency when the system is idle. While this allows the system to adjust power consumption accordingly with respect to system load, it does so at the expense of latency between frequency switching. As such, latency can offset any performance versus power saving benefits offered by the Ondemand governor if the system switches between idle and heavy workloads too often.

For most systems, the Ondemand governor can provide the best compromise between heat emission, power consumption, performance, and manageability. When the system is only busy at specific times of the day, the Ondemand governor will automatically switch between maximum and minimum frequency depending on the load without any further intervention.

cpufreq_userspace

The Userspace governor allows userspace programs (or any process running as root) to set the frequency. This governor is normally used along with the cpuspeed daemon. Of all the governors, Userspace is the most customizable; and depending on how it is configured, it can offer the best balance between performance and consumption for your system.

cpufreq_conservative

Like the Ondemand governor, the Conservative governor also adjusts the clock frequency according to usage (like the Ondemand governor). However, while the Ondemand governor does so in a more aggressive manner (that is from maximum to minimum and back), the Conservative governor switches between frequencies more gradually.

This means that the Conservative governor will adjust to a clock frequency that it deems fitting for the load, rather than simply choosing between maximum and minimum. While this can possibly provide significant savings in power consumption, it does so at an ever greater latency than the Ondemand governor.

Note

You can enable a governor using cron jobs. This allows you to automatically set specific governors during specific times of the day. As such, you can specify a low-frequency governor during idle times (for example after work hours) and return to a higher-frequency governor during hours of heavy workload.

When using the above command, be sure to remove the .ko filename suffix.

Important

When choosing an appropriate CPUfreq driver, always choose acpi-cpufreq over p4-clockmod. While using the p4-clockmod driver reduces the clock frequency of a CPU, it does not reduce the voltage. acpi-cpufreq, on the other hand, reduces voltage along with CPU clock frequency, allowing less power consumption and heat output for each unit reduction in performance.

You can also view which governors are available for use for a specific CPU using:

cpupower frequency-info --governors

Some CPUfreq governors may not be available for you to use. In this case, use modprobe to add the necessary kernel modules that enable the specific CPUfreq governor you wish to use. These kernel modules are available in /lib/modules/[kernel version]/kernel/drivers/cpufreq/.

Procedure 3.2. Enabling a CPUfreq Governor

If a specific governor is not listed as available for your CPU, use modprobe to enable the governor you wish to use:

modprobe [governor]

Example 3.1. Enabling a Governor

If the ondemand governor is not available for your CPU, use the following command:

modprobe cpufreq_ondemand

Once a governor is listed as available for your CPU, you can enable it using:

cpupower frequency-set --governor [governor]

3.2.3. Tuning CPUfreq Policy and Speed

Once you have chosen an appropriate CPUfreq governor, you can view CPU speed and policy information with the cpupower frequency-info command and further tune the speed of each CPU with options for cpupower frequency-set.

For cpupower frequency-info, the following options are available:

--freq — Shows the current speed of the CPU according to the CPUfreq core, in KHz.

--hwfreq — Shows the current speed of the CPU according to the hardware, in KHz (only available as root).

--driver — Shows what CPUfreq driver is used to set the frequency on this CPU.

--policy — Shows the range of the current CPUfreq policy, in KHz, and the currently active governor.

--hwlimits — Lists available frequencies for the CPU, in KHz.

For cpupower frequency-set, the following options are available:

--min <freq> and --max <freq> — Set the policy limits of the CPU, in KHz.

Important

When setting policy limits, you should set --max before --min.

--freq <freq> — Set a specific clock speed for the CPU, in KHz. You can only set a speed within the policy limits of the CPU (as per --min and --max).

--governor <gov> — Set a new CPUfreq governor.

Note

If you do not have the cpupowerutils package installed, CPUfreq settings can be viewed in the tunables found in /sys/devices/system/cpu/[cpuid]/cpufreq/. Settings and values can be changed by writing to these tunables. For example, to set the minimum clock speed of cpu0 to 360 KHz, use:

echo 360000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq

3.3. CPU Monitors

cpupower features a selection of monitors that provide idle and sleep state statistics and frequency information and report on processor topology. Some monitors are processor-specific, while others are compatible with any processor. Refer to the cpupower-monitor(1) man page for details on what each monitor measures and which systems they are compatible with.

Use the following options with the cpupower monitor command:

-l — list all monitors available on your system.

-m <monitor1>, <monitor2> — display specific monitors. Their identifiers can be found by running -l.

command — display the idle statistics and CPU demands of a specific command.

3.4. CPU Power Saving Policies

Allows software on supported Intel processors to more actively contribute to determining the balance between optimum performance and saving power. This does not override other power saving policies. Assigned values range from 0 to 15, where 0 is optimum performance and 15 is optimum power efficiency.

By default, this option applies to all cores. To apply it only to individual cores, add the --cpu <cpulist> option.

--sched-mc <0|1|2>

Restricts the use of power by system processes to the cores in one CPU package before other CPU packages are drawn from. 0 sets no restrictions, 1 initially employs only a single CPU package, and 2 does this in addition to favouring semi-idle CPU packages for handling task wakeups.

--sched-smt <0|1|2>

Restricts the use of power by system processes to the thread siblings of one CPU core before drawing on other cores. 0 sets no restrictions, 1 initially employs only a single CPU package, and 2 does this in addition to favouring semi-idle CPU packages for handling task wakeups.

3.5. Suspend and Resume

When a system is suspended, the kernel calls on drivers to store their states and then unloads them. When the system is resumed, it reloads these drivers, which attempt to reprogram their devices. The drivers' ability to accomplish this task determines whether the system can be resumed successfully.

Video drivers are particularly problematic in this regard, because the Advanced Configuration and Power Interface (ACPI) specification does not require system firmware to be able to reprogram video hardware. Therefore, unless video drivers are able to program hardware from a completely uninitialized state, they may prevent the system from resuming.

Red Hat Enterprise Linux 6 includes greater support for new graphics chipsets, which ensures that suspend and resume will work on a greater number of platforms. In particular, support for NVIDIA chipsets has been greatly improved; in particular for the GeForce 8800 series.

3.6. Tickless Kernel

Previously, the Linux kernel periodically interrupted each CPU on a system at a predetermined frequency — 100 Hz, 250 Hz, or 1000 Hz, depending on the platform. The kernel queried the CPU about the processes that it was executing, and used the results for process accounting and load balancing. Known as the timer tick, the kernel performed this interrupt regardless of the power state of the CPU. Therefore, even an idle CPU was responding to up to 1000 of these requests every second. On systems that implemented power saving measures for idle CPUs, the timer tick prevented the CPU from remaining idle long enough for the system to benefit from these power savings.

The kernel in Red Hat Enterprise Linux 6 runs tickless: that is, it replaces the old periodic timer interrupts with on-demand interrupts. Therefore, idle CPUs are allowed to remain idle until a new task is queued for processing, and CPUs that have entered lower power states can remain in these states longer.

3.7. Active-State Power Management

Active-State Power Management (ASPM) saves power in the Peripheral Component Interconnect Express (PCI Express or PCIe) subsystem by setting a lower power state for PCIe links when the devices to which they connect are not in use. ASPM controls the power state at both ends of the link, and saves power in the link even when the device at the end of the link is in a fully powered-on state.

When ASPM is enabled, device latency increases because of the time required to transition the link between different power states. ASPM has three policies to determine power states:

default

sets PCIe link power states according to the defaults specified by the firmware on the system (for example, BIOS). This is the default state for ASPM.

powersave

sets ASPM to save power wherever possible, regardless of the cost to performance.

ASPM support can be enabled or disabled by the pcie_aspm kernel parameter, where pcie_aspm=off disables ASPM and pcie_aspm=force enables ASPM, even on devices that do not support ASPM.

ASPM policies are set in /sys/module/pcie_aspm/parameters/policy, but can be also specified at boot time with the pcie_aspm.policy kernel parameter, where, for example, pcie_aspm.policy=performance will set the ASPM performance policy.

Warning

If pcie_aspm=force is set, hardware that does not support ASPM can cause the system to stop responding. Before setting pcie_aspm=force, ensure that all PCIe hardware on the system supports ASPM.

3.8. Aggressive Link Power Management

Aggressive Link Power Management (ALPM) is a power-saving technique that helps the disk save power by setting a SATA link to the disk to a low-power setting during idle time (that is when there is no I/O). ALPM automatically sets the SATA link back to an active power state once I/O requests are queued to that link.

Power savings introduced by ALPM come at the expense of disk latency. As such, you should only use ALPM if you expect the system to experience long periods of idle I/O time.

This mode sets the link to its lowest power state (SLUMBER) when there is no I/O on the disk. This mode is useful for times when an extended period of idle time is expected.

medium_power

This mode sets the link to the second lowest power state (PARTIAL) when there is no I/O on the disk. This mode is designed to allow transitions in link power states (for example during times of intermittent heavy I/O and idle I/O) with as small impact on performance as possible.

medium_power mode allows the link to transition between PARTIAL and fully-powered (that is "ACTIVE") states, depending on the load. Note that it is not possible to transition a link directly from PARTIAL to SLUMBER and back; in this case, either power state cannot transition to the other without transitioning through the ACTIVE state first.

max_performance

ALPM is disabled; the link does not enter any low-power state when there is no I/O on the disk.

To check whether your SATA host adapters actually support ALPM you can check if the file /sys/class/scsi_host/host*/link_power_management_policy exists. To change the settings simply write the values described in this section to these files or display the files to check for the current setting.

Important

Setting ALPM to min_power or medium_power will automatically disable the "Hot Plug" feature.

3.9. Relatime Drive Access Optimization

The POSIX standard requires that operating systems maintain file system metadata that record when each file was last accessed. This timestamp is called atime, and maintaining it requires a constant series of write operations to storage. These writes keep storage devices and their links busy and powered up. Since few applications make use of the atime data, this storage device activity wastes power. Significantly, the write to storage would occur even if the file was not read from storage, but from cache. For some time, the Linux kernel has supported a noatime option for mount and would not write atime data to file systems mounted with this option. However, simply turning off this feature is problematic because some applications rely on atime data and will fail if it is not available.

The kernel used in Red Hat Enterprise Linux 6 supports another alternative — relatime. relatime maintains atime data, but not for each time that a file is accessed. With this option enabled, atime data is written to the disk only if the file has been modified since the atime data was last updated (mtime), or if the file was last accessed more than a certain amount of time ago (by default, one day).

By default, all file systems are now mounted with relatime enabled. You can suppress it for any particular file system by mounting that file system with the norelatime option.

3.10. Power Capping

Red Hat Enterprise Linux 6 supports the power capping features found in recent hardware, such as HP Dynamic Power Capping (DPC), and Intel Node Manager (NM) technology. Power capping allows administrators to limit the power consumed by servers, but it also allows managers to plan data centers more efficiently, because the risk of overloading existing power supplies is greatly diminished. Managers can place more servers within the same physical footprint and have confidence that if server power consumption is capped, the demand for power during heavy load will not exceed the power available.

HP Dynamic Power Capping

Dynamic Power Capping is a feature available on select ProLiant and BladeSystem servers that allows system administrators to cap the power consumption of a server or a group of servers. The cap is a definitive limit that the server will not exceed, regardless of its current workload. The cap has no effect until the server reaches its power consumption limit. At that point, a management processor adjusts CPU P-states and clock throttling to limit the power consumed.

Dynamic Power Capping modifies CPU behavior independently of the operating system, however, HP's integrated Lights-Out 2 (iLO2) firmware allows operating systems access to the management processor and therefore applications in user space can query the management processor. The kernel used in Red Hat Enterprise Linux 6 includes a driver for HP iLO and iLO2 firmware, which allows programs to query management processors at /dev/hpilo/dXccbN. The kernel also includes an extension of the hwmonsysfs interface to support power capping features, and a hwmon driver for ACPI 4.0 power meters that use the sysfs interface. Together, these features allow the operating system and user-space tools to read the value configured for the power cap, together with the current power usage of the system.

Intel Node Manager imposes a power cap on systems, using processor P-states and T-states to limit CPU performance and therefore power consumption. By setting a power management policy, administrators can configure systems to consume less power during times when system loads are low, for example, at night or on weekends.

3.11. Enhanced Graphics Power Management

Red Hat Enterprise Linux 6 saves power on graphics and display devices by eliminating several sources of unnecessary consumption.

LVDS reclocking

Low-voltage differential signaling (LVDS) is a system for carrying electronic signals over copper wire. One significant application of the system is to transmit pixel information to liquid crystal display (LCD) screens in notebook computers. All displays have a refresh rate — the rate at which they receive fresh data from a graphics controller and redraw the image on the screen. Typically, the screen receives fresh data sixty times per second (a frequency of 60 Hz). When a screen and graphics controller are linked by LVDS, the LVDS system uses power on every refresh cycle. When idle, the refresh rate of many LCD screens can be dropped to 30 Hz without any noticeable effect (unlike cathode ray tube (CRT) monitors, where a decrease in refresh rate produces a characteristic flicker). The driver for Intel graphics adapters built into the kernel used in Red Hat Enterprise Linux 6 performs this downclocking automatically, and saves around 0.5 W when the screen is idle.

Enabling memory self-refresh

Synchronous dynamic random access memory (SDRAM) — as used for video memory in graphics adapters — is recharged thousands of times per second so that individual memory cells retain the data that is stored in them. Apart from its main function of managing data as it flows in and out of memory, the memory controller is normally responsible for initiating these refresh cycles. However, SDRAM also has a low-power self-refresh mode. In this mode, the memory uses an internal timer to generate its own refresh cycles, which allows the system to shut down the memory controller without endangering data currently held in memory. The kernel used in Red Hat Enterprise Linux 6 can trigger memory self-refresh in Intel graphics adapters when they are idle, which saves around 0.8 W.

GPU clock reduction

Typical graphical processing units (GPUs) contain internal clocks that govern various parts of their internal circuitry. The kernel used in Red Hat Enterprise Linux 6 can reduce the frequency of some of the internal clocks in Intel and ATI GPUs. Reducing the number of cycles that GPU components perform in a given time saves the power that they would have consumed in the cycles that they did not have to perform. The kernel automatically reduces the speed of these clocks when the GPU is idle, and increases it when GPU activity increases. Reducing GPU clock cycles can save up to 5 W.

GPU powerdown

The Intel and ATI graphics drivers in Red Hat Enterprise Linux 6 can detect when no monitor is attached to an adapter and therefore shut down the GPU completely. This feature is especially significant for servers which do not have monitors attached to them regularly.

3.12. RFKill

Many computer systems contain radio transmitters, including Wi-Fi, Bluetooth, and 3G devices. These devices consume power, which is wasted when the device is not in use.

RFKill is a subsystem in the Linux kernel that provides an interface through which radio transmitters in a computer system can be queried, activated, and deactivated. When transmitters are deactivated, they can be placed in a state where software can reactive them (a soft block) or where software cannot reactive them (a hard block).

The RFKill core provides the API for the subsystem. Kernel drivers that have been designed to support RFkill use this API to register with the kernel, and include methods for enabling and disabling the device. Additionally, the RFKill core provides notifications that user applications can interpret and ways for user applications to query transmitter states.

The RFKill interface is located at /dev/rfkill which contains the current state of all radio transmitters on the system. Each device has its current RFKill state registered in sysfs. Additionally, RFKill issues uevents for each change of state in an RFKill-enabled device.

rfkill is a command-line tool with which you can query and change RFKill-enabled devices on the system. To obtain the tool, install the rfkill package.

Use the command rfkill list to obtain a list of devices, each of which has an index number associated with it, starting at 0. You can use this index number to tell rfkill to block or unblock a device.

Example 3.2. Blocking a RFKill-enabled Device

Run the following command to block the first RFKill-enabled device on the system:

rfkill block 0

You can also use rfkill to block certain categories of devices. This command blocks all Wi-Fi devices on the system:

rfkill block wifi

To block all RFKill-enabled devices, run:

rfkill block all

To unblock RFKill-enabled devices, execute:

rfkill unblock

To obtain a full list of device categories that rfkill can block, run:

rfkill help

For more information on rkfill, see the rfkill(1) man page.

3.13. Optimizations in User Space

Reducing the amount of work performed by system hardware is fundamental to saving power. Therefore, although the changes described in Chapter 3, Core Infrastructure and Mechanics permit the system to operate in various states of reduced power consumption, applications in user space that request unnecessary work from system hardware prevent the hardware from entering those states. During the development of Red Hat Enterprise Linux 6, audits were undertaken in the following areas to reduce unnecessary demands on hardware:

Reduced wakeups

Red Hat Enterprise Linux 6 uses a tickless kernel (refer to Section 3.6, “Tickless Kernel”), which allows the CPUs to remain in deeper idle states longer. However, the timer tick is not the only source of excessive CPU wakeups, and function calls from applications can also prevent the CPU from entering or remaining in idle states. Unnecessary function calls were reduced in over 50 applications.

Reduced storage and network I/O

Input or output (I/O) to storage devices and network interfaces forces devices to consume power. In storage and network devices that feature reduced power states when idle (for example, ALPM or ASPM), this traffic can prevent the device from entering or remaining in an idle state, and can prevent hard drives from spinning down when not in use. Excessive and unnecessary demands on storage have been minimized in several applications. In particular, those demands that prevented hard drives from spinning down.

Initscript audit

Services that start automatically whether required or not have great potential to waste system resources. Services instead should default to "off" or "on demand" wherever possible. For example, the BlueZ service that enables Bluetooth support previously ran automatically when the system started, whether Bluetooth hardware was present or not. The BlueZ initscript now checks that Bluetooth hardware is present on the system before starting the service.

Chapter 4. Use Cases

This chapter describes two types of use case to illustrate the analysis and configuration methods described elsewhere in this guide. The first example considers typical servers and the second is a typical laptop.

4.1. Example — Server

A typical standard server nowadays comes with basically all of the necessary hardware features supported in Red Hat Enterprise Linux 6. The first thing to take into consideration is the kinds of workloads for which the server will mainly be used. Based on this information you can decide which components can be optimized for power savings.

Regardless of the type of server, graphics performance is generally not required. Therefore, GPU power savings can be left turned on.

ondemand governor turned on, because the last few percent of CPU performance are not important.

no disk or network plugins for tuned.

network speed should not be limited, because mail is often internal and can therefore benefit from a 1 Gbit/s or 10 Gbit/s link.

Fileserver

Fileserver requirements are similar to those of a mailserver, but depending on the protocol used, might require more CPU performance. Typically, Samba-based servers require more CPU than NFS, and NFS typically requires more than iSCSI. Even so, you should be able to use the ondemand governor.

Directory server

A directory server typically has lower requirements for disk I/O, especially if equipped with enough RAM. Network latency is important although network I/O less so. You might consider latency network tuning with a lower link speed, but you should test this carefully for your particular network.

4.2. Example — Laptop

One other very common place where power management and savings can really make a difference are laptops. As laptops by design normally already use drastically less energy than workstations or servers the potential for absolute savings are less than for other machines. When in battery mode, though, any saving can help to get a few more minutes of battery life out of a laptop. Although this section focuses on laptops in battery mode, you can use some or all of those tunings while running on AC power as well.

Savings for single components usually make a bigger relative difference on laptops than they do on workstations. For example, a 1 Gbit/s network interface running at 100 Mbits/s saves around 3–4 watts. For a typical server with a total power consumption of around 400 watts, this saving is approximately 1 %. On a laptop with a total power consumption of around 40 watts, the power saving on just this one component amounts to 10 % of the total.

Specific power-saving optimizations on a typical laptop include:

Configure the system BIOS to disable all hardware that you do not use. For example, parallel or serial ports, card readers, webcams, WiFi, and Bluetooth just to name a few possible candidates.

Dim the display in darker environments where you do not need full illumination to read the screen comfortably. Use System+Preferences → Power Management on the GNOME desktop, Kickoff Application Launcher+Computer+System Settings+Advanced → Power Management on the KDE desktop; or gnome-power-manager or xbacklight at the command line; or the function keys on your laptop.

Use the laptop-battery-powersave profile of tuned-adm to enable a whole set of power-saving mechanisms. Note that performance and latency for the hard drive and network interface are impacted.

Additionally (or alternatively) you can perform many small adjustments to various system settings:

use the ondemand governor (enabled by default in Red Hat Enterprise Linux 6)

enable laptop mode (part of the laptop-battery-powersave profile):

echo 5 > /proc/sys/vm/laptop_mode

increase flush time to disk (part of the laptop-battery-powersave profile):

Appendix A. Tips for Developers

Every good programming textbook covers problems with memory allocation and the performance of specific functions. As you develop your software, be aware of issues that might increase power consumption on the systems on which the software runs. Although these considerations do not affect every line of code, you can optimize your code in areas which are frequent bottlenecks for performance.

Some techniques that are often problematic include:

using threads.

unnecessary CPU wake-ups and not using wake-ups efficiently. If you must wake up, do everything at once (race to idle) and as quickly as possible.

A.1. Using Threads

It is widely believed that using threads makes applications perform better and faster, but this is not true in every case.

Python

Python uses the Global Lock Interpreter, so threading is profitable only for larger I/O operations. Unladen-swallow is a faster implementation of Python with which you might be able to optimize your code.

Perl

Perl threads were originally created for applications running on systems without forking (such as systems with 32-bit Windows operating systems). In Perl threads, the data is copied for every single thread (Copy On Write). Data is not shared by default, because users should be able to define the level of data sharing. For data sharing the threads::shared module has to be included. However, data is not only then copied (Copy On Write), but the module also creates tied variables for the data, which takes even more time and is even slower. For more information, see Things you need to know before programming Perl ithreads.

C

C threads share the same memory, each thread has its own stack, and the kernel does not have to create new file descriptors and allocate new memory space. C can really use the support of more CPUs for more threads. Therefore, to maximize the performance of your threads, use a low-level language like C or C++. If you use a scripting language, consider writing a C binding. Use profilers to identify poorly performing parts of your code. See also Of Programmers And Hardware: Transcending The Gap.

A.2. Wake-ups

Many applications scan configuration files for changes. In many cases, the scan is performed at a fixed interval, for example, every minute. This can be a problem, because it forces a disk to wake up from spindowns. The best solution is to find a good interval, a good checking mechanism, or to check for changes with inotify and react to events. Inotify can check variety of changes on a file or a directory.

The advantage of this approach is the variety of checks that you can perform.

The main limitation is that only a limited number of watches are available on a system. The number can be obtained from /proc/sys/fs/inotify/max_user_watches and although it can be changed, this is not recommended. Furthermore, in case inotify fails, the code has to fall back to a different check method, which usually means many occurrences of #if #define in the source code.

For more information on inotify, refer to the inotify(7) man page.

A.3. Fsync

Fsync is known as an I/O expensive operation, but this is is not completely true.

Firefox used to call the sqlite library each time the user clicked on a link to go to a new page. Sqlite called fsync and because of the file system settings (mainly ext3 with data-ordered mode), there was a long latency when nothing happened. This could take a long time (up to 30 seconds) if another process was copying a large file at the same time.

However, in other cases, where fsync wasn't used at all, problems emerged with the switch to the ext4 file system. Ext3 was set to data-ordered mode, which flushed memory every few seconds and saved it to a disk. But with ext4 and laptop_mode, the interval between saves was longer and data might get lost when the system was unexpectedly switched off. Now ext4 is patched, but we must still consider the design of our applications carefully, and use fsync as appropriate.

The following simple example of reading and writing into a configuration file shows how a backup of a file can be made or how data can be lost:

Legal Notice

This document is licensed by Red Hat under the Creative Commons Attribution-ShareAlike 3.0 Unported License. If you distribute this document, or a modified version of it, you must provide attribution to Red Hat, Inc. and provide a link to the original. If the document is modified, all Red Hat trademarks must be removed.

Red Hat, as the licensor of this document, waives the right to enforce, and agrees not to assert, Section 4d of CC-BY-SA to the fullest extent permitted by applicable law.

Red Hat, Red Hat Enterprise Linux, the Shadowman logo, JBoss, OpenShift, Fedora, the Infinity logo, and RHCE are trademarks of Red Hat, Inc., registered in the United States and other countries.

Linux® is the registered trademark of Linus Torvalds in the United States and other countries.

Java® is a registered trademark of Oracle and/or its affiliates.

XFS® is a trademark of Silicon Graphics International Corp. or its subsidiaries in the United States and/or other countries.

MySQL® is a registered trademark of MySQL AB in the United States, the European Union and other countries.

Node.js® is an official trademark of Joyent. Red Hat Software Collections is not formally related to or endorsed by the official Joyent Node.js open source or commercial project.

The OpenStack® Word Mark and OpenStack logo are either registered trademarks/service marks or trademarks/service marks of the OpenStack Foundation, in the United States and other countries and are used with the OpenStack Foundation's permission. We are not affiliated with, endorsed or sponsored by the OpenStack Foundation, or the OpenStack community.